I finished filling in a long form on a website, finally ready to preview. It was OK to go, but before I posted I had to write down the partially obscured and skewed letters and numbers in a box above. Then I clicked “post.” Red type appeared saying I had written a figure incorrectly and another set of figures appeared – even more distorted than before. This time I requested another set of figures and the whole form disappeared. The first time that’s happened.
So now I had to start over. This charming set of twisted words that block or enable entries on websites is called CAPCHAs, invented to keep out spammers. The term CAPTCHA was first used by computer scientists at Carnegie Mellon University in 2000.
CAPTCHA is actually an acronym that stands for Completely Automated Public Turing test to tell Computers and Humans Apart. That’s a pretty straight-forward title, except for the Turing test part. What exactly is a Turing test? Alan Turing was a computer theorist who invented the Turing test which humans use to see if a machine can converse like a human being. A CAPTCHA is actually an inverted Turing test whereby a machine tests to see if you are human or not, but the core principle remains.
You may wonder why CAPTCHAs don’t use images of things other than letters, like a beach or a dog, but images are harder to have an exact answer for. A picture of a beach could generate a wide variety of responses–sea, sand, sunny, ocean, and so on–but a CAPTCHA that uses letters is paired to a particular answer. Letters, unlike images, are able to be deciphered by the human eye and programmed precisely by whoever creates the CAPTCHA.
NOTE: These last two paragraphs are from http://blog.dictionary.com/captcha/